Lignocellulosic Nanocrystals from Sawmill Waste as Biotemplates for Free-Surfactant Synthesis of Photocatalytically Active Porous Silica

This work presents a new approach for more effective valorization of sawmill wastes (Beech and Cedar sawdusts), which were used as new sources for the extraction of lignin-containing and lignin-free cellulose II nanocrystals (L-CNCs and CNCs). It was shown that the properties of the extracted nanocrystals depend on the nature of the used sawdust (softwood or hardwood sawdusts). L-CNCs and CNCs derived from Beech fibers were long and thin and also had a higher crystallinity, compared with those obtained from Cedar fibers. Thanks to their interesting characteristics and their high crystallinity, these nanocrystals have been used without changing their surfaces as template cores for nanostructured hollow silica-free-surfactant synthesis for photocatalysis to degrade methylene blue (MB) dye. The synthesis was performed with a simple and efficient sol–gel method using tetraethyl orthosilicate as the silica precursor followed by calcination at 650 °C. The obtained materials were denoted as B/L-CNC/nanoSiO2, B/CNC/nanoSiO2, C/L-CNC/nanoSiO2, and C/CNC/nanoSiO2, when the used L-CNC and CNC cores are from Beech and Cedar, respectively. By comprehensive analysis, it was demonstrated that the nanostructured silica were quite uniform and had a similar morphology as the templates. Also, the pore sizes were closely related to the dimensions of L-CNC and CNC templates, with high specific surface areas. The photocatalytic degradation of MB dye was about 94, 98, 74, and 81% for B/L-CNC/nanoSiO2, B/CNC/nanoSiO2, C/L-CNC/nanoSiO2, and C/CNC/nanoSiO2, respectively. This study provides a simple route to extract L-CNCs and CNCs as organic templates to prepare nanostructured silica. The different silica structures showed excellent photodegradation of MB.


I.1. Delignification using alkaline pulping
The pre-prepared sawdust fibres from softwood and hardwood were first subjected to an alkaline pulping treatment (cooking) in order to remove hemicellulose, lignin, and other impurities by saponification and cleavage of lignin−carbohydrate linkages. This delignification stage was accomplished using an alkaline leaching of NaOH/Na2S. The mixture was transferred into a 10liter electrically heated and thermostatically controlled rotary digester with a heating jacket and a special agitation system (cooking reactor) containing 500 g of each sawdust separately. The amount of cooking liquor was calculated and 25% alkali charge was used for the pulping processes based on the oven dry weight of raw material. The percentage of Na 2 S used was 0.1%.
The considered factors were: cooking time of 45 min, temperature 165 •C, and liquid/solid ratio of 10. After the alkali treatment process, the insoluble residue (black-brown fibers) were filtered and then washed several times using distilled water in a hydra-pulper to eliminate completely the black liquor from pulp and the moisture content was measured. The obtained cooked pulp (Kraft pulp) was freeze-dried for 73 h prior any further analysis.

I.2. Bleaching process
Although, the most of the lignin and hemicellulose was removed during the alkaline pretreatment step, the remaining part of non-cellulosic compounds was removed using the bleaching method.
To do this, a part of each dried cooked pulp was separately subjected to a bleaching process by adding equal parts of buffer solution (27 g NaOH and 75 ml glacial acetic acid, diluted to 1 L of distilled water) and aqueous chlorite (1.7 wt % in water). The mixture was being boiled at 80 °C using a silicon oil bath for 2 h under mechanical stirring (fiber to liquor ratio of 1: 20). After this treatment, the bleached fibres were allowed to cool down, and were subsequently filtered and washed thoroughly using excess distilled water until the pH of the filtrate was neutral. The bleached fibers were pure white in color. Finally, the obtained cellulose was freeze-dried for 72 h and stored in polyethylene bags until its use.

I.3. Lignocellulose and cellulose nanocrystals isolation
The cooked and bleached pulps were subjected to a mechanical and acid hydrolysis process to obtain colloidal suspensions of lignin-containing cellulose nanocrystals (L-CNCs) and lignin free cellulose nanocrystals (CNCs) respectively. These processes were performed to each Kraft and bleached pulps extracted from Cedar and Beech sawdusts for the first time.

a. Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS)
The surface morphology of raw, cooked and bleached fibres obtained at the different steps of the chemical processes as well as the elemental composition of nanostructured porous silica were evaluated by scanning electron microscope-energy dispersive spectroscopy (SEM-EDS), using a LEO Gemini 1530 SEM (Germany). Each sample was dried and placed in metallic holder, and coated by a thin layer of carbon to promote the conductivity and analyzed using an accelerating voltage of 5 kV.

b. Transmission Electronic Microscopy (TEM)
The morphological properties (structure and size distribution) of lignocellulose and cellulose nanocrystals isolated from each cooked and bleached pulp-fiber successively as well as the nanostructured silica were examined by transmission electronic microscopy (TEM), using a JEM-1400 Plus TEM, JEOL Ltd., (Japan). The homogenized L-CNCs, CNCs and nanostructured porous silica suspensions were diluted to 0.01 % by mixing with Milli-Q water under ultrasonication for 10 min. About 20 µL of each diluted suspension were dropped and deposited separately onto a carbon coated copper grid using a pipette, the grid was air dried at room temperature for 3 min and the excessive water was drained with a filter paper. Then the grid was negatively stained with a 2 % (w/w) solution of uranyl acetate and dried at room temperature for 1 min in the case of L-CNCs and CNCs samples, the redundant liquid was drawn away using a filter paper also before the TEM analysis with an accelerating voltage of 80 kV.
The dimensions of whiskers (lengths and width) were determined using digital image analyses (ImageJ). A hundred nanorods were randomly selected and a minimum of 100 measurements were used to determine the average length and the diameter, respectively.

c. Particle size distribution and zeta potential measurement
The particle size distribution and the surface charge of the particles (zeta potential) of the L-CNCs and CNCs and nanostructured silica suspensions were measured using a Malvern Zetasizer 3000 (Malvern, United Kingdom). For this measurement, a capillary cell was used and the L-CNCs, CNCs and nanostructured porous silica suspensions were diluted to a concentration of 0.01 % (w/v) with Milli-Q water and sonicated in an ultrasonic bath for 30 min before being analyzed. Each sample was tested in triplicate, while 15 runs were performed for every test, and the average values were reported.

d. Fourier Transform Infrared (FTIR) Spectroscopy analysis
Untreated, alkali-treated, bleached, and acid-hydrolyzed fibres samples as well as silica-coated L-CNCs, CNCs and nanostructured silica were analysed by FTIR using a Nicolet iS50 FTIR spectrometer (USA) equipped with a Specac Golden Gate single-reflection ATR (attenuated total reflection) accessory, in order to examine any changes in functional groups during each treatment in the samples. The chemical groups for each freeze-dried and finely grounded sample were analyzed at 2 cm -1 resolution within the scanning range of wavenumber 400-4000 cm -1 and 32 accumulated scans were acquired and co-added in order to achieve an acceptable signal-to-noise ratio. OMNIC 4.0 software was applied to track the significant peaks transmittance positions at a particular wavenumber, the spectra were recorded in transmittance band mode.

e. X-ray diffraction (XRD) analysis
The raw, cooked, bleached and hydrolyzed samples as well as silica-coated L-CNCs, CNCs and nanostructured silica were subjected to a XRD analysis using an X-ray diffractometer (Bruker Discover D8, Germany) (wavelength of 1.5406 A). Each dried material in the form of milled powder was pressed into flattened sheets on a sample holder to obtain total and uniform X-ray exposure. Samples were scanned at room temperature with monochromatic Cu Kα radiations source (λ=0.15406 nm) operated at voltage of 40 kV and current of 30 mA. XRD data were collected within a diffraction angle (2θ) range of 10°-40° at scanning speed of 2 °C/min.

f. Differential thermal analysis and thermogravimetric analysis (DTA/TGA)
A TA Instruments SDT Q600 Simultaneous Thermogravimetric Analyzer was used to characterize the thermal stability of the L-CNCs, CNCs, silica-coated L-CNCs or CNCs and nanostructured silica samples (measuring the mass transformation as a function of temperature).
Approximately 5-10 mg of each sample were heated from room temperature (=23 •C) to 700 °C at a heating rate of 10 °C/min using alumina crucibles without lid. All of the measurements were performed under air atmosphere. These results confirmed the obtained yields of pulps after alkali and bleaching stages of both Cedar and Beech sawdusts in regard to the initial weight of raw and cooked fibers successively.

II.1. Preparation of cellulosic fibres  Morphological analyses of raw, cooked and bleached pulps
These yields reached 50 % and 95 % for cooked and bleached pulps extracted from Beech sawdust respectively and 38 % and 89 % for those extracted from Cedar sawdust respectively.
The low yields of cooked pulps confirm the dissolution of the majority of lignin and hemicellulose during the alkali treatment.

 Fourier Transform Infrared (FTIR) Spectroscopy analysis
FTIR spectroscopy is an efficient and relatively easy technique extensively used in cellulose research; it aims to monitor the chemical structural changes that occurred during the entire isolation process and to evaluate the structure of each obtained material by identifying the functional groups present in their surfaces [8]. The recorded spectra of raw, cooked and bleached pulps extracted from Cedar and Beech sawdusts are shown in Figure S3.
It is clearly observed that the spectra of both treated pulps (cooked and bleached) present differences compared to those of the raw samples in both cases (Cedar and Beech sawdusts).
These differences manifested in the disappearance of some characteristic bands assigned to noncellulosic compounds such as hemicelluloses and lignin [9]. As the main constituents in wood sawdust are cellulose, hemicellulose and lignin, the spectra of both raw sawdusts ( Figure S3-a) showed markedly the typical band patterns of these lignocellulosic compounds.
As can be seen, FTIR spectra of all untreated and treated samples from Cedar and Beech bending vibration, the intensity of all these peaks decreased after the alkali treatment, whereas they have completely disappeared after the bleaching stage, and further indicating the partial and the totally removal of lignin during the cooking and bleaching treatment successively [13,16].
Likewise, the three common absorption bands observed at 1421, 1364, 1314 cm -1 were mainly assigned to CH 2 symmetric bending, C-O symmetric stretching and C-H bending of cellulose respectively, the intensity of these bands increase after chemical treatment confirming the extraction of typical cellulose [21]. While the peak around 1264 cm -1 present only in the spectra of raw sawdusts corresponds to guaiacyl ring breathing, C-O out of plane stretching vibration of the aryl group in lignin C-O linkage in guaiacyl aromatic methoxy groups [16,22,23]. This band significantly decreased after chemical treatments, confirming also the removal of most lignin during the chemical treatments.
Furthermore, the obtained spectra showed a slight increase in the bands intensities observed between 1157 and 500 cm −1 after chemical treatments, proving the removal of hemicellulose and lignin species [24]. The prominent absorption bands found at 1157, 1022 are attributed to C-O-C asymmetric stretching and C−O−C pyranose ring skeletal vibration of the cellulose molecule successively, these peaks became sharper and narrower and their intensities were gradually increased from untreated sawdusts to cellulose fibers as the cellulose content of the treated fiber getting increased during the chemical treatments [16,24,25]. Typically, the structure of cellulose II was confirmed by the enhancing of the intensity of the peak observed at 1022 cm -1 [20].
Additionally, the band at 894 cm -1 was assigned to the glycosidic C-H deformation with ring vibration contribution and OH bending, which is characteristic of glycosidic linkages between glucoses in cellulose [26,27], the growth of this peak showed the increase in the percentage of cellulosic components. The band observed at 664 cm -1 is attributed to the out of plane deformation of C−H functional group, and the peak at 611 cm −1 indicates C-OH out of plane bending vibration [28].
These results indicate that the cellulose component was not removed throughout the chemical treatments preformed on Cedar and Beech sawdusts, it is the common product found in all samples. However, FTIR spectra agree with the effective removal of hemicellulose and lignin during the purification process.

 X-ray diffraction (XRD) analysis
Contrary to hemicelluloses and lignin which are totally amorphous polymers, cellulose has both amorphous and crystalline states in nature; according to Zhang and Lynd [29], the crystalline fraction of cellulose is formed due to the hydrogen bonding interactions and Van der Waals forces between adjacent molecules. X-ray diffraction (XRD) analysis was carried out to evaluate and follow the evolution of crystallinity of the Cedar and Beech sawdusts before and after different chemical treatment stages. Figure S4 shows the diffraction patterns of raw, alkali treated and bleached Cedar and Beech sawdusts successively. The crystallinity index and crystallites diameters values were presented in table S1.
After alkali treatment, clear significant changes were observed in the peak positions of both cooked Cedar and Beech pulps. These observations indicate that a polymorph conversion of cellulose I to cellulose II has occurred throughout the cooking stage under the applied conditions [35,36]. As it is seen from the diffractograms of cooked pulps from both Cedar and Beech sawdusts, three well-defined and higher intensity peaks appeared and dominated at 2θ around 12°, 20° and 22°, had Miller indices of (101), (10-1) and (002) successively, indicating the crystallographic form of cellulose II as shown in literature [37,38]. In the case of cooked pulp originated from Beech sawdust, we noticed the appearance of two other weak peaks at 2θ=15.1° and 16.5°, which correspond to the (101), (10-1) assigned to cellulose I, which is still remain after the alkali treatment, these later were resulted by the breakup of peak at 2θ= 16.21° characteristic of cellulose I in raw Beech. However, when the source material was Cedar sawdust, the cooked pulp displays only the characteristic profile of cellulose II after alkali treatment. All these peaks appeared after the alkali treatment in both Cedar and Beech cases remain present upon bleaching step and become more defined as expected, because of the progressive removal of all non-cellulosic compounds after the bleaching treatment [30,34]. This finding is further confirmed by measuring the crystallinity index (CrI) of all studied samples. The crystallinity of bleached Beech is higher than that of bleached Cedar due to the dense structure, lower lignin and hemicellulose content of cellulose hardwood fibers [39].
It is assumed that the extracted bleached pulps from Cedar and Beech are composed of pure cellulose and its CrI were measured at 74 % and 76 % successively, which are higher than that measured for the raw samples, indicating that non-cellulosic compounds were totally removed after the alkali and bleaching treatments [9]. Indeed, the applied alkali treatment was effective to partially remove such non-cellulosic compounds, because the measured CrI of the alkali treated was found to be lower than that of bleached pulps and higher than that of raw sawdusts. Similar results were also reported by R. Moriana et al. [40] who observed an increase in crystallinity index after extraction of cellulose from different forests residues [41]. This increase could be attributed to the removal of amorphous lignin and hemicelluloses as well as the rearrangement of cellulose chains [42,43].